The present embodiments relate to medical imaging. In medical imaging, the interior of a patient is scanned. Various approaches are possible, such as magnetic resonance (MR), computed tomography (CT), x-ray, fluoroscopy, ultrasound, positron emission tomography (PET), or single photon emission computed tomography (SPECT). Three-dimensional (3D) visualization is a common practice to assess and record the internal conditions of patients. For 3D visualization, volumetric effects model a wide variety of natural and non-natural phenomena, both for real-time and offline visualization. 3D visualization may be difficult for medical imaging, particularly where the volume data from the scan is rendered with an added surface (e.g., a surface representing a model of an object in the scan volume of the patient).
In terms of high-performance real-time rendering, rasterization is the most common hardware accelerated technique. Rasterization may not be suitable for rendering volumetric data and even advanced surface rendering effects (e.g., accurate lighting, transparency, shadows, etc.) remain challenging. On the other hand, offline rendering systems often use ray tracing, path tracing, and other advanced light transport simulations that naturally model the propagation of light within the model and lead to more realistic results with less development effort. These projection rendering approaches operate at a higher computational cost, but may be used for interactive and even real-time visualization.
Rendering of surfaces embedded in a volume remains a challenging problem in the traditional volume visualization systems. A number of techniques exist which implement specific types of effects near the intersections of the two types of data (e.g. surface transparency or ambient occlusions). For opaque surfaces, existing renderers may use rasterization and apply the resulting depth buffer during volume integration. The resulting rendered volume is then composited over the rasterization result. Multi-pass rendering techniques such as depth peeling provide support for embedded transparent surfaces at a significant performance cost. There exist techniques to partially model the interaction between the volume data and the surface data for a specific visual effect, such as volumetric shadows or ambient occlusions. The separately handled volume and surface data require complex computations during rendering.
While existing ray tracing renderers often support natural phenomena volumetric effects such as fog, clouds, and fire, this is generally not sufficient for the critical analysis of medical or simulation data. The performance of many such systems is also not suitable for interactive or real-time visualization.